This invention relates to multiple mode waveguides and methods of making them.
The continually increasing amount of traffic that communications systems are required to handle has forced the development of high capacity systems. Even with the increased capacity made available by systems operating between 10.sup.9 Hz and 10.sup.12 Hz, traffic growth is so rapid that saturation of such systems is anticipated in the very near future. Higher capacity communication systems operating around 10.sup.15 Hz are needed to accommodate future increases in traffic. These systems are referred to as optical communication systems since 10.sup.15 Hz is within the frequency spectrum of light. Conventional electrically conductive waveguides which have been employed at frequencies between 10.sup.9 and 10.sup.12 Hz are not satisfactory for transmitting information at frequencies around 10.sup.15 Hz.
The transmitting media required in the transmission of frequencies around 10.sup.15 Hz are hereinafter referred to as "optical waveguides".
U.S. Pat. Nos. 3,711,626--Keck and Schultz; 3,823,995--Carpenter; and 3,737,293--Maurer disclose methods of making optical waveguides.
Light propagates through optical waveguides in one or more transmission modes depending upon the radius, relative indices of refraction of cores and cladding and the wavelength of the light. Generally, it is desirable to limit propagation to one particular mode. However, this requires the use of an extremely small diameter waveguide. It is desirable to use a larger diameter waveguide in order to more easily apply light thereto and to connect sections of wave guide. Larger diameter waveguides will sustain propagation in more than one mode.
"THEORY OF DIELECTRIC OPTICAL WAVEGUIDES" by Marcuse, Academic Press, New York and London, 1974 describes the theory of mode coupling. U.S. Pat. Nos. 3,666,348--Marcatilli and 3,687,514--Miller et al discuss increasing the band pass of an optical waveguide through mode coupling. Whereas individual uncoupled modes have a large group velocity dispersion, coupling locks the energy flow of these modes together and decreases pulse spreading.
Mode coupling can be explained in several ways, one of which is set forth in the aforementioned Miller et al patent. Another simplified explanation of mode coupling is that the photons of light jump back and forth between the different modes which are coupled. Each mode has a characteristic velocity of propagation. Photons which jump back and forth between two modes arrive at the end of the waveguide with a characteristic velocity which is an average of the propagation velocities of the two modes in which they traveled.
The aforementioned Marcatilli and Miller et al patents disclose the use of diameter and axis perturbations to achieve mode coupling. Bumps on the core-cladding interface or bubbles in the waveguide are also perturbations which cause mode coupling. Manufacturing control of these perturbations to obtain the desired coupling properties is difficult.
The paper entitled "MODE COUPLING IN GRADED-INDEX FIBERS" which was presented at the Symposium on Optical and Acoustical Micro-Electronics, Polytechnical Institute of New York, Apr. 16-18, 1974, Robert Olshansky and application Ser. No. 725,172, Robert Olshansky filed Sept. 21, l976 describe the use of gradient index perturbations to achieve mode coupling.